(a) Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a liquid crystal display which bears uniform brightness characteristic over the entire screen area without leakage of light.
(b) Description of the Related Art
Generally, a liquid crystal display has an upper substrate with a common electrode and color filters (usually called the “color filter substrate”), a lower substrate with thin film transistors and pixel electrodes (usually called the “TFT array substrate”), and a liquid crystal layer sandwiched between the color filter substrate and the TFT array substrate. Different electrical potentials are applied to the pixel electrodes and the common electrode while forming electric fields to change the liquid crystal molecule orientation. In this way, the light transmission is controlled to display picture images.
In such a liquid crystal display, with only a TFT attached to it, the charge applied to switch the liquid crystal leaks away in a brief time after a signal arrives. Therefore, it is necessary to connect an additional capacitor to the liquid crystal so that the liquid crystal is able to retain the charge associated with the first signal until a second signal is received.
For example, a capacitance may be conferred on a liquid crystal by using an adjacent gate electrode line.
Each pixel electrode overlaps over the previous gate line with an insulating layer interposed to form a storage capacitance Cst. The pixel electrode faces the common electrode with a liquid crystal layer interposed to form a liquid crystal capacitance Clc. Furthermore, a parasitic capacitance Cgd is formed between a gate electrode and a drain electrode.
The voltage applied between each pixel electrode and the common electrode changes at 60 Hz (60 frames per second). Within one frame, pulses of Von are applied to sequentially turn on TFTs from the first gate line to the last gate line. In case the Von pulse is applied to a particular gate line, off-voltages Voff are applied to the other gate lines. When the voltage applied to the common electrode Vcom is set to 5V, the on-voltage Von becomes to be about 20V, and the off-voltage Voff to be about −7V. When the on-voltage Von is applied to a particular gate line, the TFTs positioned at that line are in an on-state, and the picture signal voltages applied to the data lines are transmitted to the pixel electrodes. In contrast, when the off-voltage Voff is applied to the TFTs at the particular gate line while applying Von to the previous gate line, the electric potential Vg of the previous gate line is elevated by 27V from −7V to 20V. At this time, the electric potential of the pixel electrode Vp is also increased. The amount of increased potential of the pixel electrode ΔVp can be expressed by the following equation:ΔVp=[Cst/(Cst+Clc+Cgd+other parasitic capacitance)]×ΔVg(=27V).
At this time, the liquid crystal capacitance Clc as a function of voltage difference between Vcom and Vp as well as the parasitic capacitance Cgd are varied together. Thereafter, when the previous gate line is shifted from Von to Voff, the electric potential of the pixel electrode Vp returns to the initial state, but not the liquid crystal capacitance Clc and the parasitic capacitance Cgd due to voltage dependence thereof. As such a variation in electrical potentials of the pixel electrodes at the second to last gate lines are made in the same pattern, the pixel electrode portions at the second to last gate lines bears uniform brightness at the same gray scale. However, since the pixel electrodes at the first gate line have no previous gate line, the electrical potentials of those electrodes change in different manner and the brightness becomes different in the same gray scale. As the brightness at the first gate line portion is usually brighter compared to other gate line portions, the picture images displayed at that portion disturbs the human eye.
In order to solve such a problem, the technique of adding a storage capacitor line G0 and connecting it to the second gate line G2 or the last gate line Gm has been proposed. However, when the G0 line is connected to the G2 line, the integrated circuit (IC) for driving the G2 line should be also employed for driving the G0 line, resulting in shortage in driving current. Accordingly, the second gate line portion becomes much brighter than other gate line portions at a normally white mode. This phenomenon becomes serious as the electric load applied to each gate line becomes greater with the trend of high definition and increased screen size. In contrast, when the G0 line is connected to the Gm line, complicated wiring via printed circuit boards (PCBs) should be made to interconnect the G0 line and the Gm line, and the first and the last gate line portions differs in brightness from the other gate line portions.
In the meantime, the TFT array substrate usually has a size larger than the color filter substrate. In this connection, when assembled, the periphery of the TFT array substrate without the corresponding black matrix portion is exposed to the outside so that light leaks at the exposed portion.